In North American freight railroad service, conventional three-piece freight car trucks having two wheelsets have evolved to satisfy a variety of important operating and economic requirements. Among other requirements, they must be capable of safely supporting and equalizing very high wheel loads over a wide range of track conditions while delivering a high level of economic value to the railroads that use them. In addition to those basic criteria, the trucks and their parts must be interchangeable throughout the system of interconnected railroad networks. The three-piece trucks in service today have, to a large extent, met these requirements because their general designs are simple, flexible, durable and reliable. However, in this evolutionary process a major aspect of truck design for performance efficiency has been largely ignored, design for warp stiffness.
When a conventional three-piece truck encounters sufficient energy in the course of its normal use, usually due to high speed operation, the wheelsets are forced to move laterally relative to the track and relative to one another, causing the instability known as truck hunting. Truck hunting is undesirable, because it causes high lateral forces to be imparted to the rail vehicle and its lading, and because it produces increased drag on the locomotive, resulting in reduced efficiency. Likewise, when a conventional three-piece truck encounters a curve in the normal course of its use, the wheelsets are often forced to move laterally relative to one another, resulting in a condition known as truck warp. Truck warp is undesirable because it causes a high angle of attack to arise between the leading wheelset and the rail, resulting in high rates of wear on the rails and wheels. Whether they are a result of high speed or curving, truck hunting and truck warp are generally characterized by a lateral displacement of the wheelsets relative to one another and a change of the square relationship of the axles relative to the side frames into an angular relationship.
The recent testing of conventional three-piece freight car truck designs has shown that a large proportion of the interaxle shear stiffness which governs their performance is attributable to the side frame pedestal to roller bearing adapter connection. However, the current standard design of this connection has an inherent problem in that it only provides resistance to unsquaring movements between the side frames and wheelsets by means of coulomb friction. Theoretical modeling and real track testing have proven that, in terms of warp stiffness, friction alone is not sufficient to produce optimum efficiency in curving and stability performance. Rather, optimum performance requires that a constant linear spring stiffness exist, in addition to the friction characteristic, between the wheelsets to resist their relative lateral movement.
The side frame to roller bearing adapter connection design is generally characterized by a roller bearing adapter in a loosely fit upside down U-shaped pedestal jaw which allows the relative freedom of the side frame to rotate in yaw and roll with respect to the roller bearing adapter. The connection is comprised of a flat bearing surface on the side frame end, the pedestal, which bears on an arcuate upper bearing surface on the roller bearing adapter, the crown. The connection is completed by a pair of pedestal jaws, one fore and one aft of the roller bearing adapter, each having on its surface a thrust lug for bearing the longitudinal and lateral forces of the roller bearing adapter relative to the side frame. This connection is specified by AAR standards to have a minimum gap between the vertical surfaces of 1/16. Therefore, it forms a loose connection that allows the side frame to rotate in the horizontal plane and roll in the vertical plane relative to the roller bearing adapter. In part, the pedestal connection is designed this way in order to ensure a uniform load distribution on the roller bearing for maximum durability and reliability. However, it is this gap fit connection and the lack of a yaw spring stiffness between the side frame and axle that is the fundamental problem with the interaxle shear stiffness of the three-piece truck.
Another important aspect of the three-piece truck frame is the connection between the roller bearing adapter and the roller bearing. This connection is generally characterized by a very close, uniform fit. Specified in AAR standards, this connection ensures that loads on the roller bearing are evenly distributed and that the roller bearing does not move relative to the roller bearing adapter. As a result, the roller bearing adapter moves rigidly with the roller bearing which moves with the axle.
Prior art structures describing connections between the truck frame and the journal box, journal box adapter or roller bearing adapter exist in different forms and they vary in their configurations and their intended purposes. One prior art structure in particular, Rossell U.S. Pat. No. 2,782,732, describes a device which has as its objective to fix a plate in a pedestal jaw by means of two parallel longitudinal links and one lateral link as a frictional interface interposed between the journal box and suspension element. The described purpose of the prior art structure was to "impose a heavy frictional resistance to the journal boxes in order to increase high speed stability by breaking up the harmonic axle motions which cause hunting." While the Rossell invention may have been effective at improving high speed stability in a box frame truck, it would not be effective at increasing warp stiffness in a three-piece truck.
The usefulness of the prior art structure in Rossell is limited in that it would only be effective and useful on a box frame truck with a primary suspension. As opposed to a three-piece truck, a box frame truck has an integrally cast rectangular unit frame that encompasses and rests on a suspension above the wheelsets'journals. Unlike the three-piece truck, the box frame truck has an inherent warp stiffness, because the basic frame is one large cast piece. When attached to a box frame, the Rossell three link structure would effectively restrain the described friction plate against lateral and longitudinal movement. In a three-piece truck, however, the three link structure would have no effect on warp stiffness because the link structure is designed to resist translation and would not effectively resist the relative yaw movements that occur between the side frame and roller bearing adapter. This is because Rossell describes a link that is connected from the truck frame to the roller bearing adapter with single point, flexible, jointed ends which can only resist forces in tension and compression and not in rotation.
Another aspect of the prior art in Rossell is that it describes a structure that connects the truck frame with a friction plate that is interposed between the journal box and the suspension element. In the modern three-piece truck, however, the roller bearing adapter and the roller bearing have such a close fit that they are the functional equivalent of the journal boxes of the old technology. Therefore, the friction plate described in Rossell is not the functional equivalent of the roller bearing adapter. Rather, it is the functional equivalent of a wear plate interposed, in the three-piece truck, between the roller bearing adapter crown and the side frame pedestal. Such a structure, in the three-piece truck, would have no effect whatsoever.